change the type of the argument of `drop_in_place` lang item to `&mut _` by WaffleLapkin · Pull Request #154327 · rust-lang/rust

WaffleLapkin · 2026-03-24T17:44:25Z

We used to special case core::ptr::drop_in_place when computing LLVM argument attributes with this hack:

Lines 383 to 392 in db5e2dc

    
           let drop_target_pointee_info = drop_target_pointee.and_then(|pointee| { 
        
               assert_eq!(pointee, layout.ty.builtin_deref(true).unwrap()); 
        
               assert_eq!(offset, Size::ZERO); 
        
               // The argument to `drop_in_place` is semantically equivalent to a mutable reference. 
        
               let mutref = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, pointee); 
        
               let layout = cx.layout_of(mutref).unwrap(); 
        
               layout.pointee_info_at(&cx, offset) 
        
           }); 
        
           if let Some(pointee) = drop_target_pointee_info.or_else(|| layout.pointee_info_at(&cx, offset))

This is because even though drop_in_place takes a *mut T it is semantically a &mut T (remember how &mut Self is passed to Drop::drop). This is apparently relevant for perf.

This PR replaces this hack with a simpler solution -- it makes drop_in_place a thin wrapper around newly added core::ptr::drop_glue, which is the actual lang item and takes a &mut T:

rust/library/core/src/ptr/mod.rs

Lines 810 to 833 in d2563d5

    
           pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) 
        
           where 
        
               T: [const] Destruct, 
        
           { 
        
               // Due to historic reasons, `drop_in_place` takes a pointer rather than a reference, 
        
               // which results in worse codegen since we don't apply noalias/dereferenceable llvm 
        
               // attributes to pointer arguments. To workaround this without breaking public 
        
               // interface, `drop_in_place` calls the lang item, rather than being one directly. 
        
               // SAFETY: 
        
               // - compiler glue has the same safety requirements as this function 
        
               // - the pointer must be valid as per the safety requirement of this function 
        
               unsafe { drop_glue(&mut *to_drop) } 
        
           } 
        
           /// Helper function for `drop_in_place`. 
        
           #[lang = "drop_glue"] 
        
           const unsafe fn drop_glue<T: PointeeSized>(_: &mut T) 
        
           where 
        
               T: [const] Destruct, 
        
           { 
        
               // Code here does not matter - this is replaced by the 
        
               // real drop glue by the compiler. 
        
           }

The rest of the PR is blessing tests and cleaning up things which are not necessary after this change.

One thing that is a bit awkward is that now that drop_glue is the actual lang item, a lot of the comments referring to drop_in_place are outdated. Should I try fixing that?

I've also changed async_drop_in_place to take a &mut T, and it simplified the code handling it a bit. (since it's unstable we don't need to introduce a wrapper)

cc @RalfJung
Closes #154274

RalfJung · 2026-03-24T17:47:02Z

Oh wow I hadn't expected my wish to be fulfilled before I could even finish my PR that motivated me to express the wish in the first place. :-) ❤️

RalfJung · 2026-04-06T13:52:26Z

I like the interpreter changes and renames. Not sure what the status of the rest of the PR is, but if you submit just those as a separate PR I'll r+.

…s, r=RalfJung Slightly refactor mplace<->ptr conversions split off of rust-lang#154327 r? RalfJung

WaffleLapkin · 2026-05-06T11:59:24Z

@bors try jobs=test-various

change the type of the argument of `drop_in_place` lang item to `&mut _` try-job: test-various

rust-bors · 2026-05-06T14:12:22Z

☀️ Try build successful (CI)
Build commit: 006163a (006163a6720d54bc0cb68c169754981f750a7f87, parent: e95e73209faf6ead2bc5c7636e45e589a751b79b)

scottmcm · 2026-05-06T21:49:15Z

@bors r=RalfJung,scottmcm,saethlin

rust-bors · 2026-05-06T21:49:18Z

📌 Commit 94e4ddf has been approved by RalfJung,scottmcm,saethlin

It is now in the queue for this repository.

rust-bors · 2026-05-07T04:05:36Z

☀️ Test successful - CI
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-log-analyzer · 2026-05-07T04:08:06Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

rust-bors · 2026-05-07T04:16:31Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T04:31:29Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T04:46:30Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T05:01:30Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-timer · 2026-05-07T05:11:56Z

Finished benchmarking commit (4ddd453): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	2
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Max RSS (memory usage)

Results (primary -1.2%, secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.1%	[1.5%, 2.7%]	2
Regressions ❌ (secondary)	4.4%	[1.2%, 7.9%]	4
Improvements ✅ (primary)	-3.4%	[-4.2%, -2.1%]	3
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-1.2%	[-4.2%, 2.7%]	5

Cycles

Results (primary 7.3%, secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	7.3%	[5.8%, 8.8%]	2
Regressions ❌ (secondary)	0.8%	[0.4%, 2.3%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.3%, -3.3%]	1
All ❌✅ (primary)	7.3%	[5.8%, 8.8%]	2

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	2
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	2
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.0%]	4
Improvements ✅ (secondary)	-0.2%	[-1.9%, -0.0%]	19
All ❌✅ (primary)	-0.1%	[-0.3%, 0.2%]	6

Bootstrap: 500.816s -> 499.861s (-0.19%)
Artifact size: 395.23 MiB -> 394.98 MiB (-0.06%)

rust-bors · 2026-05-07T05:16:30Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T05:31:30Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T05:46:30Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T06:01:31Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T06:16:36Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

rust-bors · 2026-05-07T06:31:31Z

☀️ Test successful - CI, Post merge analysis
Approved by: RalfJung,scottmcm,saethlin
Duration: 3h 12m 34s
Pushing 4ddd453 to main...

WaffleLapkin · 2026-05-07T14:07:24Z

@Kobzol uhhhhh... it's bors supposed to do this?

bjorn3 · 2026-05-07T14:14:51Z

https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Bors.20posting.20success.20message.20a.20dozen.20times/with/593477072

RalfJung reviewed Mar 24, 2026

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Comment thread compiler/rustc_const_eval/src/interpret/place.rs

Comment thread library/core/src/ptr/mod.rs Outdated

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rustbot added A-run-make Area: port run-make Makefiles to rmake.rs A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` T-rust-analyzer Relevant to the rust-analyzer team, which will review and decide on the PR/issue. labels Apr 5, 2026

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WaffleLapkin mentioned this pull request Apr 6, 2026

Slightly refactor mplace<->ptr conversions #154894

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JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Apr 6, 2026

Rollup merge of rust-lang#154894 - WaffleLapkin:mplace_ptr_conversion…

d1e186f

…s, r=RalfJung Slightly refactor mplace<->ptr conversions split off of rust-lang#154327 r? RalfJung

WaffleLapkin added 2 commits May 6, 2026 13:57

bless codegen-units tests

278517f

make NonNull::drop_in_place call drop_glue directly

94e4ddf

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rust-bors Bot pushed a commit that referenced this pull request May 6, 2026

Auto merge of #154327 - WaffleLapkin:drop_in_place_ref, r=<try>

006163a

change the type of the argument of `drop_in_place` lang item to `&mut _` try-job: test-various

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 6, 2026

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Nadrieril mentioned this pull request May 7, 2026

[POC] create an MVP for using Destruct for custom dtors #156090

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Kobzol mentioned this pull request May 7, 2026

Do not trust GitHub's PR state after the PR has been already merged rust-lang/bors#752

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icmccorm mentioned this pull request May 7, 2026

Emit retags in codegen to support BorrowSanitizer (part 2) #156210

Open

	let drop_target_pointee_info = drop_target_pointee.and_then(\|pointee\| {
	assert_eq!(pointee, layout.ty.builtin_deref(true).unwrap());
	assert_eq!(offset, Size::ZERO);
	// The argument to `drop_in_place` is semantically equivalent to a mutable reference.
	let mutref = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, pointee);
	let layout = cx.layout_of(mutref).unwrap();
	layout.pointee_info_at(&cx, offset)
	});

	if let Some(pointee) = drop_target_pointee_info.or_else(\|\| layout.pointee_info_at(&cx, offset))

	pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T)
	where
	T: [const] Destruct,
	{
	// Due to historic reasons, `drop_in_place` takes a pointer rather than a reference,
	// which results in worse codegen since we don't apply noalias/dereferenceable llvm
	// attributes to pointer arguments. To workaround this without breaking public
	// interface, `drop_in_place` calls the lang item, rather than being one directly.

	// SAFETY:
	// - compiler glue has the same safety requirements as this function
	// - the pointer must be valid as per the safety requirement of this function
	unsafe { drop_glue(&mut *to_drop) }
	}

	/// Helper function for `drop_in_place`.
	#[lang = "drop_glue"]
	const unsafe fn drop_glue<T: PointeeSized>(_: &mut T)
	where
	T: [const] Destruct,
	{
	// Code here does not matter - this is replaced by the
	// real drop glue by the compiler.
	}

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Overall result: ❌✅ regressions and improvements - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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WaffleLapkin commented Mar 24, 2026 •

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